Acceleration switches are designed to issue a signal when a threshold acceleration value is detected. Acceleration switches are widely used in air bag systems to detect the sudden deceleration of a vehicle during crash. Acceleration switches are usually mechanical switches having a spring-loaded mass that makes contact with an electrode, thus acting as a switch, when subjected to acceleration greater than a predetermined threshold value of acceleration. Today, acceleration switches are commonly manufactured using micromachining.
U.S. Pat. No. 6,336,658B1 discloses an acceleration switch having a first and second inertia mass where the second inertia mass is lighter than the first inertia mass. A pair of torsion bars connect to the first inertia mass and act as a turning shaft for the first inertia mass. The acceleration switch also has a pair of electrodes formed on a substrate facing the second inertia mass. When acceleration of a predetermined value or greater is applied to the acceleration switch, the second inertia mass moves into contact with the pair of electrodes to act as a switch.
U.S. Pat. No. 6,236,005B1 discloses an acceleration switch having an inertia mass and an electrode element that are pivoted about an axis of a spring element that connects the inertia mass and the electrode element to a housing. When the switch is subject to acceleration greater than a predetermined value, an electrical contact point on the electrode element touches a corresponding contact area in the housing, so that an electrical signal is provided to indicate that the predetermined value of acceleration is exceeded.
The threshold value of acceleration will depend on the physical parameters of the switches such as the stiffness of the torsion bar or the spring element, the inertia mass, and the distance between the contacting electrodes. The threshold value of the switches described in these publications is generally difficult to adjust once the switches are made.
To overcome the limitation in adjusting the threshold value, various configurations of acceleration switch are proposed where the threshold value is adjustable by changing electrode voltage. U.S. Pat. No. 5,905,241 discloses an acceleration switch that uses snap-through buckling phenomenon of a bimorph beam to displace a movable electrode into contact with another electrode at a fixed distance when a threshold acceleration force is experienced. EP Patent Application 0924730A1 discloses an acceleration switch equipped with an inertial mass that is deflectable from a holding position to an actuated position in response to a predetermined acceleration force. A voltage controlled hold plate generates an electrostatic force that opposes the acceleration forces and holds the inertial mass in the holding position until overcome by the predetermined acceleration force. In both cases, the predetermined acceleration force necessary to trigger the switch can be adjusted by adjusting the electrode voltage or the voltage applied to the hold plate.
However, the above switches involve at least one electrical contact that is formed with metal and may be subjected to problems such as micro-welding, arcing, and oxidation, which may cause failure of the switches.
US patent application Ser. No. 2004/0161869A1 discloses a contactless acceleration switch without metal contacts as shown in FIG. 1. The acceleration switch 100 comprises a mass 112 attached to a spring 114, a substrate layer 102 with a threshold adjustment channel 108 located between a source 104 and a drain 106 thereon, and a gate insulating layer 120 located above the substrate layer 102,. The threshold adjustment channel 108, the source 104, and the drain 106 are implanted in the substrate layer 102 positioned at a predetermined distance from the mass 112. The mass 112 operates as a moveable gate in combination with the source 104 and the drain 106 to form a field effect transistor (FET). When acceleration level exceeds a threshold acceleration value, an electric field may form between the mass 112 and the substrate layer 102, creating an electrostatic force that attract the mass 112 to the substrate layer 102. The mass 112 moves towards the substrate layer 102 to a position of critical distance at which point the electrostatic force exceeds a spring force created by the spring 114. The mass 112 may suddenly contact the gate insulating layer 120 and the strength of the electric field reaches a maximum level, thus inverting the threshold adjustment channel 108 and allowing current to flow between the source 104 and the drain 106, which turn on the FET. One problem with the switch 100 is that a substantially constant voltage in the range from less than five volts for low acceleration, to hundreds of volts for large acceleration range devices, has to be applied between the mass 112 and the substrate layer 102.
It is an object of the invention to provide an acceleration-sensitive switch that overcomes or ameliorates one or more of the disadvantages of the prior art or which at least provides a useful alternative.